The American Civil War was a war fought between the northern states and southern states of USA. In 1961, Abraham Lincoln was elected as the President of USA. Lincoln was against slavery and supported a strong federal government.
The southern states, on the other hand, strongly supported slavery and believed that individual states should have more power. This disagreement with the Presidency of Abraham Lincoln led the southern states to secede from the Union and ultimately led to a war between the two sides.
Slavery was one of the most divisive issues between the northern and southern states. The southern states had a largely rural economy and relied on farming. And they used a huge number of slaves to do the labor on the farming lands. So the southern states supported slavery and believed that it was crucial to their economy.
The northern states, on the other hand, were more industrialized and slavery was illegal in the northern states. The problem was that the northern states had more population and more votes. This meant that if they wanted to outlaw slavery, they could do so even if the southern states opposed them.
The southern states believed that individual states should make their own laws and that the federal government should have only a few basic powers. The northern states, on the other hand, wanted the federal government to be stronger.
The southern states were wary of this because they knew that the northern states had more votes and were more dominant in the federal government. So they knew that if the federal government grew stronger, the northern states would also grow strong and the rights of southern states would no longer be protected.
During the 19th century, USA was rapidly expanding towards the West. As a result, new territories were being added to USA and new states were being born. The problem, the northern states wanted these new states to be slavery-free states while the southern states wanted them to support slavery.
If most of these states became Free states, support for slavery would have decreased. If they became slavery-supporting states, the attempts of northern states to end slavery would have suffered. So both sides fought over the new states. Ultimately, most of the new states in the West became free states and the southern states which supported slavery resented this.
Abraham Lincoln ran for Presidency as a representative of the Republican Party. The Republic Party was strongly opposed to slavery. The southern states were fiercely opposed to the Party and to Abraham Lincoln. They thought that if Lincoln succeeded, the northern states would outlaw slavery and ruin the economy of the southern states.
So when Lincoln was elected as President, the southern states thought they had enough. So starting with South Carolina, they started breaking away from the Union and announcing their independence. Eventually, 11 southern states broke away from USA and formed their own government called the Confederacy.